1. Field of the Invention
The present invention is directed to an X-ray apparatus of the type having a d.c.-a.c. converter supplied with voltage that is connected via a line connection to a high-voltage transformer that supplies the high-frequency operating voltage to an X-ray tube.
2. Description of the Prior Art
D.c.-a.c. converter circuits with which an alternating voltage is generated are utilized in X-ray units, this alternating voltage being forwarded via a line connection to the high-voltage transformer that generates the high voltage that is required for the operation of the X-ray tube. The high voltage signal at the secondary side of this transformer is subsequently also rectified, and smoothed. The d.c.-a.c. converter is usually a series resonant circuit converter that is supplied with a d.c. voltage that is usually acquired directly from the mains voltage by rectification and filtering. It contains four semiconductor switches that are conductive in pairs in alternation on the basis of a suitable drive. The line connection is usually required in X-ray embodiments wherein the high-voltage generator, which contains the high-voltage transformer and possibly other components connected following the transformer, is provided in the immediate proximity of the X-ray tube or is contained together therewith in a single tank.
A typical resonant circuit utilized for this purpose is composed of a capacitance C and an inductance L=L"sgr"+LL, wherein L"sgr" is the stray inductance of the high-voltage transformer or transformers and LL is the line inductance of the line connection to the high-voltage generator. Particularly at high d.c.-a.c. converter power and operating frequencies of 30 kHz or higher that are currently standard, it is difficult to make the resonant circuit inductance as small as is necessary. The stray inductance L"sgr" of the high-voltage transformer cannot be arbitrarily reduced in size without having the high-voltage transformer become too large. The line inductance LL makes up a substantial part of the overall inductance L, particularly given single-tank generators that require a longer cable between d.c.-a.c. converter and single tank. An overall inductance which is too high, however, has a disadvantageous effect on the operation of the X-ray apparatus.
Japanese Published Application 6-53 83 discloses an X-ray apparatus of the above-described type.
German Utility Model 85 26 448 discloses a high-voltage X-ray cable with a grid control line. The center of the cable disclosed therein has a grid control line and two electric resistance wires that are stranded with one another. A conductive band that surrounds this configuration is in turn surrounded with the actual high-voltage conductor, which in turn carries an inner conductive layer that is surrounded by an insulation. An outer conductive layer, a shielding as well as an outside insulation are also provided. Either a single high-voltage line, or two high-voltage lines as well as two electrical resistance wires that are stranded with one another are utilized in the inside of the line in the X-ray line disclosed in German Utility Model G 91 07 953. A conductive sheath is provided around the exterior, followed by a high-voltage insulation as well as another outer conductive sheath on which a shielding is disposed that in turn carries an outside cladding. German Patent 39 29 990 discloses a low-impedance, coaxial line that has a number of coaxial cables connected in parallel.
An object of the present invention is to provide an X-ray means that can also be operated at high operating frequencies and high power.
The above object is achieved in accordance with the principles of the present invention in an X-ray apparatus having a d.c.-a.c. converter circuit supplied with energy that is connected via a line connection to a high-voltage transformer which supplies high-frequency operating voltage to an x-ray tube, wherein the line connection is composed of four concentrically arranged conductors, with a first and second of these conductors carrying the operating voltage and being insulated from each other and being disposed close to each other for achieving a low line inductance, and having a ratio of conductor radius to conductor thickness to produce a low ohmic impedance. A third of these conductors serves for shielding and is insulated from the second conductor for eliminating capacitive shift currents and is connected to a point of constant potential in the d.c.-a.c. converter. A fourth of these conductors serves for shielding and is insulated from the third conductor, and is connected to the housing of the d.c.-a.c. converter circuit, as well as to the housing of the high-voltage transformer or a device component contained within the high-voltage transformer.
In the inventive X-ray apparatus, the line connection has a low line inductance, which is inventively realized by arranging the two conductors that proceed concentrically relative to one another extremely close to one another. Inventively, the line inductance can be xe2x89xa60.25 xcexcH/m, particularly xe2x89xa60.15 xcexcH/m; the spacing between the first and the second conductor should be xe2x89xa62 mm, particularly xe2x89xa61.5 mm. Even given a long length, which usually amount to approximately 12 m or more, these low inductances contribute only slightly to the overall inductance, so that this is in an acceptable range overall. Moreover, the two current-carrying conductors of the inventive line connection have a low ohmic impedance, which can be achieved by corresponding dimensioning of the conductors as to conductor radius and conductor thickness. i.e., the line connection is also optimized with respect thereto. The ohmic impedance at high frequencies is dependent on the conductor radius and on the effective conductor layer (penetration depth). As is known, the penetration depth is dependent on the conductor material and the operating frequency. The conductor thickness should not be significantly greater than the penetration depth, thereby allowing conductor material to be saved. Given copper and a frequency of 30 kHz, for example, the penetration depth amounts to barely 0.4 mm. It is therefore inventively provided that the thickness of the conductors is xe2x89xa61 mm, particularly xe2x89xa60.5 mm. The ohmic impedance should be xe2x89xa620 mxcexa9/m, particularly xe2x89xa615 mxcexa9/m in accordance with the invention. The low ohmic impedance leads to low losses.
Since, due to the skin effect, current conduction occurs only in the outer layer of conductors, the first, most interiorly disposed conductor in the invention can be a waveguide, which is advantageous in view of the lower use of material as a result thereof as well as in view of the flexibility and pliability of the line connection.
As described, a concentrically arranged, third conductor serving as shielding and insulated from the second conductor is inventively provided for carrying away capacitive shift currents. Such shift currents, otherwise would flow (drain) via the capacitance between the second conductor and the grounded metal parts located in the proximity without employing this shielding conductor, and would cause significant disturbances in the system as stray currents. These shift currents, are eliminated via this third conductor, which preferably at constant potential in the d.c.-a.c. converter for this purpose, i.e. the capacitive shift currents are returned into the d.c.-a.c. converter in this case and do not flow via the housing of the d.c.-a.c. converter or some other metallic article.
A concentrically arranged fourth conductor serving for shielding and insulated from the third conductor, is provided, with which thexe2x80x94albeit slightxe2x80x94magnetic field generated by the current through the third conductor is shielded, so that no shift currents having a disadvantageous influence are generated in this case, either. Inventively, the fourth conductor is connected to the housing of the d.c.-a.c. converter circuit and to the housing of the high-voltage transformer, or the housing of the system component in which the high-voltage transformer is arranged, and serves as a protective conductor, i.e. the fourth conductor performs a double function. It is especially advantageous that a separate protective conductor need not be provided. Inventively, the high-voltage transformer itself can be arranged in a common housing together with the X-ray tube; of course, it is also possible to arranged the high-voltage transformer and any following components, such as a high-voltage rectifier and high-voltage smoothing capacitors, away from the X-ray tube in a separate housing.
It has proven expedient when the first and second conductor, and potentially the third and/or fourth conductor as well, are inventively fashioned as stranded conductors whose individual wires are coated with a layer of silver or a silver alloy. This coating of the stranded conductors is advantageous, because a further reduction of the lead resistance (skin effect) is thereby achieved plus it affords a better gliding of the individual wires relative to one another, and a better conductor mobility. This is particularly advantageous in the embodiment wherein all four conductors of the cable, which can be referred to as xe2x80x9cquadraxial cablexe2x80x9d, are provided.
In addition to the X-ray apparatus itself, the invention is also directed to a line connection, particularly for an X-ray tube of the type described above. This line connection has four conductors insulated from one another and arranged concentrically relative to one another, with the innermost first and the second conductors serving for current conduction, the third conductor serving for shielding the inner conductor pair and the fourth conductor serves for shielding the third conductor, and the first and the second conductor are arranged so close to one another that the line inductance amounts to xe2x89xa60.25 xcexcH/m, particularly xe2x89xa60.15 xcexcH/m, and the ratio of the conductor radius to the conductor thickness in the first and second conductor is selected such that the ohmic impedance amounts to xe2x89xa620 mxcexa9m, particularly xe2x89xa615 mxcexa9m.